1. Field of the Invention
The present invention relates to fuel burners and more particularly to very high capacity fuel burners using liquid fuels or gas utilized for drying and similar processes.
2. Description of the Prior Art
Large, high capacity fuel burners are generally used in industries requiring drying of various materials. For example, such burners are required for operating large rotary aggregate dryers, and for kiln drying and processing of lime, sand, bauxite, coal, cement and the like. In the making of asphalt roads, portable drying units are used for drying the aggregate before mixing with the asphalt.
In drying aggregate, as an example of an application of the fuel burners under consideration, a typical unit may have a rotating horizontal drum 50 feet in length and 8 feet in diameter. The wet rock is introduced into one end of the drum, carried to the top of the drum and dropped back. The material is gradually carried to the opposite end of the drum and removed by a conveyor. A fuel burner, which may have an outlet chamber of from one to two feet in diameter, is placed at one end of the drum. The hot gases and air emanating from the burner are directed through the falling aggregate, known as the aggregate curtain, and serves to drive out all moisture from the material. An exhaust fan at the output end of the drum draws the heated air therethrough. The gas temperature at the burner input end may be on the order of 2400.degree. F. dropping to about 350.degree. F. at the opposite end of the drum. For large dryers, such as described above, the burners are required to produce as much as 200 million BTU per hour.
Several typical problems in prior art burners of this nature involve the control of the combustion air, optimum atomization of oil when used as a fuel, stabilization of the flame, and obtaining large turn-down ratios. To solve some of these problems, it is common in the art to use a refractory lined combustion chamber and ignition port in combination with the burner. In this type of construction, a refractory lined quarl of frusto-conical shape is disposed with its small end at the atomizer. Known as an ignition port, the quarl produces a flame front plane transverse to the axis of the cone which serves to ignite the atomized fuel from the atomizer nozzle. The burner gases and heated air move toward the outlet end of the cone with their velocity decreasing as the cross-sectional area increases. The flame front therefore stabilizes at the transverse plane of the cone at which the flame propagation speed is essentially equal to the gas and air velocity. As the amount of fuel and air is changed with desired changes in burner output, the flame stabilization plane will therefore move forward along the cone for increased output and back toward the atomizer for reduced output. For maximum output, the flame occurs near the largest cross section of the cone, producing a very large combustion volume with flames extending forward for a considerable distance beyond the ignition port. Thus, a refractory lined combustion chamber is necessary to contain this broad combustion volume. A typical burner with a capacity of 100 million BTU per hour may require an ignition port with a length of about three feet and a combustion chamber of about four feet in diameter and five feet in length. Thus, this method of flame stabilization adds greatly to the overall burner length.
Other burners utilize a bluff body for stabilization of the flame wherein air is caused to spill over the edge of a flat plate, creating a turbulence to produce an external recirculation of hot gases which tend to stabilize the flame. However, there are limits to the range of burner output over which this method is usable. For example, when the flame output is increased, the stabilization conditions are no longer in existence and the flame tends to move outward from the burner with a possible loss of ignition. Thus, this type of burner must be operated over a relatively narrow range and any short term cessation of operation requires shut off of the burner. Energy must be utilized and operation time wasted in re-heating the refractory and the like when the burner is to be restarted.
While the use of refractory lined combustion chambers and ignition ports are effective to some extent for control of combustion, this method has several serious disadvantages. For example, the refractory material gradually spalls with use and must be periodically replaced, requiring shut down of operation and causing high maintenance costs. The refractory lined chambers also greatly increase the size of the burner system which adds to the cost of portable drying systems mounted on trailers such as used in asphalt highway construction. The length of the chambers reduces the dryer barrel length that can be used on a given trailer bed size, thereby limiting the capacity of the dryer.
Some prior art burner designs for use with oil provide means for atomization of the oil in which compressed air and oil are mixed in the atomizer. When operating at low output, both air and oil pressures are reduced. Large turn-down ratios are difficult to obtain in such burners due to the necessity of designing the nozzle for optimum atomization at maximum output. When both the oil pressure and atomizing air pressure are reduced to obtain less output, the nozzle becomes less efficient and the turn-down ratio is limited by loss of good atomization of the fuel.
Thus, a need exists for a high capacity burner which has efficient atomization, which can provide a large turn-down ratio to permit idling of the burner without excessive fuel consumption, and which will have a small combustion volume not requiring a refractory lined ignition port and combustion chamber.